1. Field of the Invention
The present invention relates to a zoom lens system in which a back focal distance can be short.
2. Description of the Related Art
In a zoom lens system for a single lens reflex (SLR) camera, in order to have a space behind the photographing lens system, a retrofocus type lens systems, which includes negative first lens group (a front lens group) and a positive second lens groups (a rear lens group) in this order from an object, is generally employed so that a longer back focal distance can be provided. On the other hand, in a zoom lens system in which a back focal distance can be short (e.g., a zoom lens system in a compact camera), since there is no need to provide a longer back focal distance unlike a zoom lens system for a single lens reflex (SLR) camera, a telephoto type lens system, which includes a positive first lens group (a front lens group) and a negative lens group (a real lens group) in this order from the object, is generally employed. Because the two-lens-group zoom lens system is employed, the number of lens elements are fewer, simplification of the structure is attained, the manufacturing costs can be reduced, and the miniaturization of the lens system can be achieved.
Along with the popularization of compact cameras, not only the correcting of aberrations but also an increase of relative brightness (a smaller F-number) is required in the zoom lens system of a compact camera. In a two-lens-group zoom lens system, in order to satisfactorily correct aberrations from the short focal length extremity toward the long focal length extremity, it is necessary to correct aberrations in each lens group. However, the smaller the F-number is, the more difficult the correcting of aberrations in each lens group becomes. In particular, for correcting aberrations in the front lens group, the number of lens elements thereof have to be increased. Due to the increase of the lens elements, two major structural problems arise.
The first problem is that when the F-number is smaller the diameter of the shutter has to be made larger whereby the shutter unit itself has inevitably to be made larger. In a two-lens-group zoom lens system which performs zooming by varying the distance between the positive front lens group and the negative rear lens group, conventionally the diaphragm is provided between the front and rear lens groups from a structural point of view. However, if the above explained large-sized shutter unit as a result of a smaller F-number is positioned between the front and rear lens groups, there is a possibility that the large-sized shutter unit interferes with the rear lens group. In particular, in a collapsible barrel type zoom lens system with the large sized shutter unit, when the lens barrel is being collapsed, the front and rear lens groups cannot adequately approach each other, and the collapsed barrel length is increased. The overall thickness of the camera is then increased.
The second problem is that an increase of burden on the focusing mechanism due to an increase of the lens elements in the front lens group, which is attributable to a small F-number. Conventionally, in the two-lens-group zoom lens system which performs zooming by varying the distance between the positive front lens group and the negative rear lens group, focusing is generally performed by driving the entire front lens group. However, if the entire front lens group having an increased weight due to the increased lens elements as a result of a smaller F-number is to be driven, the focusing mechanism has to be enlarged, which hinders the miniaturization of a camera.